This invention relates to digital data transmission systems employing a radio link. More particularly, this invention relates to radio digital data links in which the frequency of the radio link shifts from one value to another in a known and periodic manner.
Frequency hopping radio links are very useful when secure radio transmissions and receptions are required. The rapid change in frequency from one known frequency to another in a seemingly random fashion almost entirely prevents either eavesdropping or jamming of the radio link. Of course, in order for the radio link to be effective, both receiver and transmitter must be synchronized in time with each other as well as each know the pseudo-random frequence of changes. Such systems have been employed by the military for many years. This military system, however, is a relatively primitive AM frequency-hopping voice radio system. In one such system, a cycle lasts approximately 62.5 milliseconds, which includes a relatively short period in which the transmission frequency shifts from one level to another in which no information may be exchanged and a relatively long remainder interval which may be used for the AM voice transmission. The relatively short "hop" interval in which the frequencies change is not detrimental to any significant degree to the transmission of audio signals representing human voice because the human ear is able to bridge over these information gaps during the "hops" with little difficulty Insofar as is known, this represents the state of the prior art. No systems are known which utilize a frequency hopping radio link to transmit digital data. This is because it would be impossible to transmit ordinary serial digital data over such a frequency hopping link because the "hop" intervals would break into the digital data stream and render the whole digital message useless. That is to say, that every bit of the digital information signal is important; portions of digital data messages may not be discarded without a total loss of information content for the entire message.